Power transmission chains are widely used in the automotive industry not only for ignition timing, but also for transferring mechanical power to the driving wheels of a vehicle.
One conventional type of power transmission chain used for automotive timing chain applications, for instance, is referred to as a “silent chain”. These chains, in general, are constructed of a plurality of interleaved sets of metallic links with adjacent sets of links joined together by pivot means. Each chain link of such silent chains has a pair of toes separated by a crotch, in which each toe is defined by an inside flank and an outside flank, with the inside flanks being joined by or at the crotch. Each such link also has a pair of apertures, which are connected by the pivot means. The pivot means used in the past in such silent chains have been metallic joint components, such as metallic roller pins or metallic rocker pins. Such silent chains and link components thereof have been adapted to be used with toothed sprockets in the power transmission assembly or arrangement. Historically, the inside flank or flanks of the links of such silent chains have been used to engage the sprockets in automotive timing or motion transferring chain applications. The metallic pins and rocker joints must be dimensioned within very tight tolerances to avoid loose connections from occurring between separate sets of links.
In the manufacture of such conventional silent chains, a preload or tensile load typically is applied after assembly of the chain and before the chain is put into service. Application of the preload is done to preadjust the length of the chain before actual loads are applied to the chain in a power transmission application. The preload not induced in the case of conventional silent chain constructions, the chains could be susceptible to relatively significant initial elongation when the chain is first placed in service, which is undesirable. However, when a preload is applied to a conventional silent chain construction, the load acts on the roller pins such that the flanks of the rocker pins press against the walls of the apertures in the link plates, thereby tending to seat the rocker pin in the aperture providing greater contact area between the rocker pins and the apertures and reducing wear. As a result, undesirable offsets of the chain pitch could occur with regard to the apertures, which could adversely affect the performance of the chain. Consequently, conventional silent chain constructions have a possible shortcoming associated with the preload requirements.
Another conventional timing chain configuration is the roller chain construction. One conventional roller chain construction generally includes a plurality of inner link plate pairs and outer link plate pairs arranged alternately in tandem and joined together in an articulated manner. To achieve such a construction, each inner link has been provided as at least two inner plates arranged substantially parallel and transversely spaced from one another. These inner plates have coaxially aligned pin holes at their respective ends for receiving a metal pin at each end thereof. A bushing is fastened, such as by press fitting, between each of the pair of holes located at the respective ends of the two inner plates of each inner link. The outer links each include at least two parallel, spaced apart outer plates that are joined to each other using a joint component such as a single round pin. To accomplish this, a pin joining two adjacent ends of the outer plates of each outer link projects through holes at associated ends of intervening inner plates of a first adjacent inner link, while a pin joining the other ends of the outer plates of the same outer link projects through holes of ends of inner plates of a second adjacent inner link. A rotatable cylindrical roller is mounted on each bushing such that the roller is located between the inner plates. Each such roller is capable of loose rotation on its associated bushing. The roller chains mesh with sprockets with their rollers drivingly engaging the flanks of the sprocket teeth.
For silent chain constructions, wear in link components can be a significant concern. Link wear develops due to the movement of the chain links under load as they engage the sprockets. Lubrication of metal chain components with oil or grease has long been used as a strategy to reduce chain wear. However, the lubricants tend to capture grit and other particulate debris that can come into contact with the chain. This can lead to the unintended effect of causing wear and abrasion on chain components. This chain wear can be even further exacerbated by other factors.
For instance, in automotive timing chain applications, increased risk of wear also has been associated with direct injection gasoline engines. The fuel formulations developed and used for that purpose are prone to undergo adverse chemical reactions with conventional chain lubricants. Among other things, these inadvertent chemical reactions make the oil more acidic, and thus potentially corrosive to metallic drive chain components contacted by the contaminated lubricant. In addition, these adverse chemical reactions degrade the overall lubricity and performance of the oil, which permits more wear to occur in chain components. Increased wear problems also have been associated with diesel engines, such as those used in passenger car applications. Namely, the use of diesel fuel in a combustion engine leads to the build-up of sulfur in the oil used to lubricate the chain, which can result in the formation of corrosive acids in the oil. These acids are corrosive to metals and thus they can chemically attack metallic drive chain components. Singly or in combination, these above-mentioned phenomena accelerate wear and degradation of timing and power transmission drive chains.
Wear, corrosion and abrasion of chain component surfaces, such as joint components, is a problem because it leads to loss of material in the chain components, especially joint components such as metal pins and bushings, and also rollers (if used). This lost material can itself further contaminate the lubricating oil, and consequently contribute to increased abrasive wear on the chain. This loss of material ultimately creates a gap between different joint components of the drive chain. These gaps cause the chain to “stretch” or increase in length from its original length. The overall tension on such stretched chains have been modified by taking-up the resulting chain slack, such as by using a conventional blade spring tensioner. In addition, the loss of material and corrosion can compromise the mechanical properties required of the chain. Moreover, the amount of wear and/or corrosion may not occur uniformly from one chain link to the next throughout the chain, such that uniform meshing engagement of the worn chain and sprockets may not be possible, even if the overall chain is re-tensioned. As a consequence, the useful life span of the chain is reduced.
The introduction of ceramic materials in specific parts of certain types of chains and chain conveyors has been generally proposed. For example, U.S. Pat. No. 5,069,331 discloses a harvester conveyor chain having composite links fitted with non-metallic overlay bushings. U.S. Pat. No. 4,911,681 discloses a ceramic conveyor belt formed of ceramic bars interconnected with ceramic spacers by ceramic rods with end fixation provided between the rods and bars, in which the bars are attached fixedly on the rods using ceramic end tabs whereby a projection in the bar fits within an end groove on the rods while leaving a space on the opposite side of the rod which is filled with ceramic putty to radially and axially fix the rods, precluding any freedom of rotation for the rods. The conveyor chains of the '331 and '681 patents concern non-articulated links of chains using rotatable pin connections. Additionally, conveyor chains, such as described in the '331 and '681 patents generally concern lower tensile load environments relative to the overall chain size involved such that longitudinal chain stretch problems are not a concern.
U.S. Pat. No. 5,829,850 discloses a track system for use in a tracked vehicle having a crawler chain and a driving sprocket wheel, which has a pin assembly with a bushing or one piece bolt the outer periphery of which is at least composed of a material consisting of silicon nitride or a zirconium oxide with at most 15% sintering additives. A track system with its relatively large dimensions of the chain components translates into a relatively low load environment without serious longitudinal chain stretch problems. U.S. Pat. No. 5,884,387 discloses a drive system having self-lubricating ceramic components, identified as center links, center rollers and sprockets. U.S. Pat. No. 5,803,852, like the '387 patent, discloses a drive system having ceramic center links, center rollers, and sprockets arranged for sliding and rotating contact. U.S. Pat. No. 4,704,098 discloses a combination link chain constructed of metallic outer link plates and plastic inner link plates.
There has been a need for drive chains to be used in high load and high speed environments such as automotive timing chain or other power transmission applications having better resistance to wear, corrosion, abrasion resistance, and longitudinal chain stretch or elongation, yet without incurring substantial sacrifices in chain performance and while being cost effective and practical from a manufacturing standpoint.